Dashpot timer having a coated metallic piston



a- 5, 1969 D. s. BREED 3,458,992

DASHPOT TIMER HAVING A COATED METALLIC PISTON Filed Dec. 1. 1967 v I a/4 L/ INVENTOR DAV/D S. BEG'ED BY AADMALQ W ATTORNEYS United StatesPatent 3,458,992 DASHPOT TIMER HAVING A COATED METALLIC PISTON David S.Breed, 57 Hoagland Ave., Rockaway, NJ. 07866 Filed Dec. 1, 1967, Ser.No. 693,686 Int. Cl. G04f N08 US. Cl. 58-144 10 Claims ABSTRACT OF THEDISCLOSURE A timing mechanism of the dashpot type wherein a cylindricalmetallic piston travels in a cylinder at a controlled rate, the pistonhaving a coating of polymeric material on the cylindrical surfaceadjacent the interior wall of the cylinder to insure a consistent rateof descent of the piston within the cylinder.

BACKGROUND AND SUMMARY OF THE INVENTION The present invention relates toa timer comprising a piston which travels in a cylinder at a controlledrate due to the predictable fluid flow rate through the predeterminedannular clearance between the piston and interior cylinder wall. Adashpot timer of this general type is further described in US. PatentNo. 3,171,245 issued on Mar. 2, 1965.

It has been proposed to dispose the piston against the wall of thecylinder by various means, such as by inclining or tilting the cylinder,in order to insure accurate reproducibility of the rate of descent ofthe piston on each run. However, in certain applications, space or otherlimitations preclude the use of the common devices to dispose the pistonagainst the cylinder wall. Also, it has been found that when spacelimitations require the use of a piston having a small axial lengthrelative to its diameter, there will be a tendency for the piston tocock in the cylinder. This inadvertent cocking in turn has an effect onthe consistency of the piston rate of descent.

In the above situations, it is desirable that some provision be made toassure that the piston will maintain the same positioning relative tothe cylinder on each run. Heretofore, no effective means to accomplishthis requirement had been devised.

The use of a metallic piston in a glass or ceramic cylinder of thedescribed type has generally been unsatisfactory since the diflerentcoeflicients of expansion between the two materials produce inconsistentresults when the dashpot is used under conditions of varyingtemperature. In particular, the different rates of expansion change theannular clearance between the piston and cylinder, which in turn effectsthe rate of fluid flow past the piston. Also, most metals are subject tocorrosion which similarly has an effect on the clearance dimension. Inaddition, the friction force between the metal and glass surfaces ishigh which tends to make the results inconsistent.

The present invention relates generally to a dashpot timer comprising acylinder filled with a fluid in either gaseous or liquid form. Acylindrical metallic piston is disposed in the cylinder, the cylindricalsurface of which is coated with a polymeric material. The miscroscopicsurface irregularities which are inherent in the polymeric coating onthe piston have been found to effectively maintain a precise andconsistent orientation of the piston in the center of the cylinder, asWell as contribute to the predictability of the fluid flow rate betweenthe piston and cylinder wall.

In the case of air or gas dashpots, the piston is made from a metallicmaterial having a coefficient of expansion somewhat less than that ofthe material from which the cylinder is made. In the case of liquiddashpots, the metallic material has a somewhat higher coefficient ofexpansion. This factor is important in compensating for the effects oftemperature variation on the accuracy of the device, as hereinafterfurther described.

It is an object of the invention to provide an accurate dashpot timerfor use in those applications where space or other limitations precludethe use of known devices for insuring the consistent positioning of thepiston within the cylinder. It is a further object of the invention toprovide a timer which is exceptionally reliable, susceptible to longlife and relatively inexpensive to manufacture. Another object is toprovide a timer having a metallic piston which is not susceptible tocorrosion and which is accurate over a wide range of temperatures.

Other objects and advantages of the present invention will becomeapparent to those skilled in the art from the following detaileddescription.

BRIEF DESCRIPTION OF THE DRAWINGS FIGURE 1 is a perspective view of thedashpot timer incorporating the teachings of the present invention;

FIGURE 2 is a horizontal sectional view thereof;

FIGURE 3 is a view taken along the line 33 of FIG- URE 2;

FIGURE 4 is an enlarged view of a portion of FIG- URE 3.

DESCRIPTION OF THE PREFERRED EMBODIMENT In FIGURES 13, a dashpot timer10 is illustrated which incorporates the teachings of the presentinvention. The timer includes a ceramic cylinder 12 in which a metallicpiston 14 is slidably disposed. The cylinder 12 may be sealed at bothends, or it may be open at one end as shown at 13. The exteriorcylindrical surface of the piston 14 has a coating of a polymericmaterial 16 applied thereto. The interior wall of the cylinder 12 andthe coated piston 14 have their mating diameters very closelydimensionally controlled to provide a determinable clearance 18 throughwhich a fluid 115, in either a gaseous or a liquid form, is adapted toflow. The determinable clearance permits the time interval required forthe longitudinal travel of the piston 14 in cylinder 12 to be accuratelycontrolled.

The fluid 15 may be a gas such as air, or a liquid such as siliconefluid. It will thus be apparent that by proper choice of fluid 15, andby proper design of the determinable clearance 18, the time delay may bevaried from a fraction of a second to several months duration.

It should be understood that a great many polymeric materials are suitedfor use as the coating 16. However, the use of a fluorocarbon polymersuch as Teflon is preferred.

The invention contemplates the use of any suitable ceramic material inthe fabrication of the cylinder. However, the use of glass is preferredsince it has many advantages, among *which are its low cost, largevolume production techniques, superior wear characteristics, corrosionresistance, and its low thermal coefficient of expansion.

The particular metal used in making the piston is of importance i-faccuracy of the timer over a Wide temperature range is to be maintained.Since the viscosity of a gas increases with the temperature, one wouldexpect the time delay of a gas filled dashpot to also increase, assumingall other factors remained constant. Also, since most metals have ahigher coefficient of expansion than glass, the clearance between thepiston and cylinder would normally be decreased thus further increasingthe time delay at higher temperatures. It has been found however, thatby choosing a metal having a coefficient of expansion less than that ofthe ceramic used for the cylinder, an increased clearance will result.By proper design, this increased clearance will permit the same flowrate of the thicker or higher viscosity gas as at the lower temperature.Specifically, the metal used in a gas filled dashpot would necessarilyhave a coefiicient of expansion in the order of 6.7)( or less inches perinches per degree Fahrenheit.

In the case of a dashpot filled with a liquid such as silicone fluid,consideration must be given to the fact that the viscosity will decreasewith an increase in temperature. Thus the metal piston will necessarilyhave a higher coeflicient of expansion than the cylinder if the timedelay is to remain constant.

Since common metals can be alloyed in such a way to create a change in acoeflicient of expansion, many different materials could be used infabricating the piston. For purposes of example, the following metalsare cited:

Tantalum, tungsten and molybdenum Age hardenable stainless steel Hightemperature steel Ultra-high strength steel Martensitic stainless steelMalleable iron Titanium alloy Low expansion nickel alloy Of these, thelow expansion nickel alloys are the most readily available in a widerange of coefficients of expansion and consequently, would be the mostlikely group to use.

(It is an important feature of the present invention to utilize themicroscopic surface irregularities (shown in exaggerated fashion inFIGURE 4) in the polymeric coating on the piston to insure thereproducibility and thus accuracy of the device. The irregularities arein the nature of myriad and resilient high spots which tend to keep thepiston centered in the cylinder. Also, the irregularities produce aserpentine or labyrinth fluid flow between the walls of the cylinder andpiston which is much more predictable and consistent in total flowvolume than is a straight line flow. These factors combine to produce ahighly accurate dashpot timer which can be utilized in many applicationsnot heretofore possible.

Since the coating thickness can be accurately controlled to within 50millionths of an inch, a relaxation of the tolerances on the piston andcylinder is permitted. This relaxation substantially reduces the cost ofmanufacturing the piston and cylinder and thus contributes to the use ofthe timer in low cost applications.

Coatings of Teflon of from .0002 to .001 inch have been successfullyapplied. Multiple coatings could, however, be applied such thatthicknesses of several thousandths of an inch could be built up whilestill maintaining the required roundness tolerances. The utilization ofthe surface irregularities in the coating film is particularlyapplicable when the clearance between the piston and cylinder measuredon the radius is between .00005 inch and .0002 inch. However, byincreasing the coating particle size, clearances somewhat larger thanthis could still make use of this advantage.

The polymeric coating may be applied by placing the piston on a mandrelwhich is rotated at a speed in the order of 100 r.p.m. The coating isapplied from a spray gun which may be operated by a solenoid for apredetermined length of time to control the depth of the coating. Inorder to insure a good adherence of the coating to the piston surface,it may be necessary to etch the surface with a suitable acid, such ashydrofluoric, before spraying.

I claim:

1. A dashpot timer of the class described, comprising a fluid filledcylinder having a precision internal cylindrical bore, and a cylindricalmetallic piston disposed in said cylinder, the cylindrical surface ofsaid piston having a polymeric coating of predetermined thicknessthereon to form a predetermined annular clearance between the piston andthe surface of said cylindrical bore, said polymeric coating includingmyriad and resilient high spots in contact with said bore surface tomaintain said piston centered in said bore and to cause a serpentinefluid flow through said annular clearance whereby a consistent rate ofdescent of the piston within the cylinder is insured.

2. A dashpot timer of the class described comprising a cylinder having aprecision interior cylindrical wall, a metallic piston slidably disposedin said cylinder, the exterior surface of said piston beingsubstantially cylindrical and having a polymeric coating thereon ofpredetermined thickness to form a predetermined annular clearancebetween the piston and the surface of said cylindrical wall, saidpolymeric coating including myriad and resilient high spots in contactwith said wall to maintain said piston centered within said cylindricalwall and to cause a serpentine fluid flow through said annular clearancewhereby a consistent rate of descent of the piston within the cylinderis insured, and a fluid filling the interior of said cylinder.

3. A dashpot timer as described in claim 2 wherein the fluid filling thecylinder is air.

4. A dashpot timer as described in claim 2 wherein the fluid filling thecylinder is silicone.

5. A dashpot timer as described in claim 3 wherein the metallic pistonhas a coeflicient of expansion less than that of the cylindrical wall ofsaid cylinder.

6. A dashpot timer as described in claim 5 wherein the cylinder is madefrom a ceramic material.

7. A dashpot timer as described in claim 6 wherein the polymeric coatingon the piston is Teflon.

8. A dashpot timer as described in claim 4 wherein the metallic pistonhas a coeflicient of expansion greater than that of the cylindrical wallof said cylinder.

9. A dashpot timer as described in claim 8 wherein the cylinder is madefrom a ceramic material.

10. A dashpot timer as described in claim 9 wherein the polymericcoating on the piston is Teflon.

References Cited UNITED STATES PATENTS 1,955,565 4/1934 Schmidt et al.92248 2,714,927 8/1955 Stern et al. 58-144 2,817,562 12/ 1957 Fleming etal 92249 3,1 1 1,0 04 11/1963 Allenbach 58144 3,171,245 3/1965 Breed58-1 RICHARD B. WILKINSON, Primary Examiner EDITH C. S IMMONS, AssistantExaminer U.S. Cl. X.R. 581; 92248

